The invention in general relates to water purification systems of the ultrafiltration class and in particular concerns apparatus for collecting the purified water emanating from the filters in such a system.
The ultrafiltration process represents an important advance in water purification technology because it permits a very high level of water purification using simple mechanical means. In this process very thin plastic membranes (about 100 millionths of an inch thick) having very tiny pores (on the order of about 10 angstroms) are used to remove from water all contaminants that have a particle size larger than the pore size. Both suspended and dissolved contaminants are removed. These characteristics of the ultrafiltration process and the nature of the materials which form the ultrafiltration membranes, create several significant problems that are not encountered in conventional water purification systems.
One important limitation on ultrafiltration systems is that the ultrafiltration membranes must constantly be maintained in a wet condition because if the membranes are allowed to dry they irreversibly consolidate and lose their permeability. Previous systems have maintained the wetness of the membranes either by continual pumping water through the membranes, or by immersion of the membranes. The former requires a constant power output which may create a significant additional operating expense. The latter method either reduces the efficiency of the filter because the water creates a back pressure on the filter or requires an immersing operation whenever the system is shut down.
The small pore size of the ultrafiltration membrane plus other characteristics of the ultrafiltration process limit the process to significantly lower flow rates through the filter than in conventional filtration systems. Thus, usually, a very large number of filters, or at least a very large area of membrane is required in a commercial scale system. The large filter area plus the rather thin filter thickness means that the probability of a leak developing somewhere in the filter is significant. Thus, it is very important in an ultrafiltration system that the individual filters or at least small groups of filters be isolable from the system as a whole so that repairs or replacements of the filter may be possible without requiring the shutdown of the entire system. In order to achieve this result ultrafiltration "filter beds" usually are divided up into a large number of units that are individually isolable and replaceable. Previous designs of such systems often required a choice between the following two alternatives: a system that had many units on one conduit, and thus when one unit failed all the units on that conduit had to be shut down while repairs and/or replacements were being performed; or a system in which each unit had its own individual piping system, which design permitted one to shut down only a minimum part of the system when a unit failed, and thus permitted more efficient operation of the system, however this system required significantly higher capital expenditures.
A characteristic of ultrafiltration systems is that the filtrate is very highly purified, thus even a small leak can significantly decrease the purified water quality. Therefore, it is highly desirable in an ultrafiltration system to have the filtrate from each group of filter units capable of being isolated so that leaks would not significantly contaminate the entire purified water system before they could be detected.
Accordingly it is an object of this invention to provide an apparatus for collecting purified water in an ultrafiltration system that overcomes one or more disadvantages of previous ultrafiltration water collection systems.
It is another object of this invention to achieve the preceding object with an ultrafiltration water collection system that maintains the ultrafilters in a wet condition without requiring continuous operation or otherwise reducing the efficiency of the filtration system.
It is a further object of this invention to achieve one or more of the preceding objects with an ultrafiltration water collecting system that permits quick and efficient isolation, repair, and replacement of individual ultrafiltration units without requiring individualized waste water input systems for each unit and at the same time minimizing the contamination of the purified water in the system before and while such repairs and replacements take place.